1. Field of the Invention
This invention relates to a method of coating an inner surface of a cathode ray tube with lining graphite, and more particularly a method of coating an inner surface of a funnel of a cathode ray tube with lining graphite which serves as an inner conductive film between a screen portion and electron gun of the cathode ray tube.
2. Description of the Prior Art
Generally, a cathode ray tube comprises a panel 1 and a funnel 2 provided with a neck portion 2a having a reduced cross-section and has an inner space 3 defined by the panel and the funnel. Furtheremore, an electron gun 4 is mounted within the neck portion 2a of the funnel to emit an electron beam 5, and a shadow mask 6 formed with small through-holes is disposed within the enlarged portion of the inner space 3 opposite the neck portion 2a through elastic support at the inner corners of the panel 1. In addition, a screen 7 having a luminous fluorescent material coated thereon in a given pattern is bonded to the inner surface of the panel 1.
With the cathode ray tube thus constructed, when the electron gun 4 mounted within the neck portion 2a is operated to emit the electron beam 5, the emitted electron beam passes through each through-hole of the shadow mask 6 and lands on the fluorescent material coated on the screen 7, thereby forming a given image on the screen. At this time, in order to enable the electron beam 5 emitted from the electron gun to be accelerated by a high voltage applied by the electron gun through the lining graphite and aluminum films (not shown) coated on the shadow mask and the screen, and thus effectively land on the screen coated with the fluorescent material, the inner space 3 of the cathode ray tube must be maintained in a high vacuum state of approximately 10.sup.-6 .about.10.sup.-7 Torr (hereinafter referred to as "the set degree of vacuum"). In a typical manufacturing process of the cathode ray tube, this vacuum is not obtained, and instead the tube has the a of vacuum only on the order of 10.sup.-4 .about.10.sup.-5 Torr.
When the inner space 3 has a degree of vacuum less than the set degree of vacuum, the electron beam 5 emitted from the electron gun 4 fails to effectively land on the screen 7. Therefore, in order to compensate for such a low degree of vacuum, as shown in FIG. 2A, a getter 10 having a frame 10a connected to a high voltage electrode 4a of the electron gun 4 is disposed within the inner space of the cathode ray tube in supported relation to the inner surface of the funnel 2, and then, the sealing process of the funnel is carried out. Thereafter, a high-frequency induction coil 9 is attached to the outer side of the funnel to carry out getter flashing by high-frequency induction heating. As a result of the flashing, the principal ingredients, such as barium, nickel, etc., of the getter 10 are dispersed to every nook and corner of the inner space 3 of the cathode ray tube to adsorb and remove foreign matters obstructing traveling of the electron beam 5.
While, in a small-sized cathode ray tube, the thus obtained vacuum satisfactorily gets to the high vacuum state having the set degree of vacuum (appoximately 10.sup.-6 .about.10.sup.-7 Torr) the flashing of only the getter 10 by the high-frequency induction heating in a large-sized cathode ray tube (of more than 25 inches) fails to satisfactorily attain the desired degree of vacuum. In view of this, in the large-sized cathoe ray tube, as shown in FIG. 2B, in addition to the getter 10 disposed on the inner surface of the funnel 2, a frittable getter 11 having a frame 11a is addittionally mounted on the inner surface of the funnel portion adjacent to the screen, thereby enabling to realize the set degree of vacuum.
In the past, high-frequency induction heating of the getter or the frittble getter disposed within the funnel by the high-frequency induction coil located outside of the funnel has been carried out with graphite coated on the entire inner surface of the funnel of the cathode ray tube as shown in FIG. 3, thereby dispersing the principal ingredients of the getter or the frittable getter. However, if the high-frequency induction heating is performed with the lining graphite coated as set forth above, the lining graphite is carbonized and the resultant waste of the carbonized graphite is moved toward the electron gun due to vibration or any other movement of the cathode ray tube, thereby giving rise to a discharge phenomenon in the cathode ray tube and thus deteriorating a withstand voltage characteristic of the high voltage side.